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27th HiLumi WP2 Task Leader Meeting
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Europe/Zurich
6/R-018 (CERN)
Minutes of the 27th WP2 Task Leader Meeting held on 10/04/2014
Present: G. Arduini, O. Brüning, R. De Maria, M. Giovannozzi, E. Métral, T. Pieloni, A. Valishev, A. Wolski.
Minutes and Follow-up of Actions (Gianluigi)
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The minutes were approved with the following comments:
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Elias reported that for the e-cloud simulations in crab cavities, codes need important development to include RF fields and it will take several months for the development. The simulations of the heat load due to image currents as a function of the temperature of operation will be available in May.
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Gianluigi noted that it would be important to have an estimate on the instability threshold in the presence of electron cloud in the triplets, matching sections and main quadrupoles assuming that cryogenics can cope with the heat load. He asked whether this could be done for the Japan meeting. Elias will verify that with Giovanni Rumolo.
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Gianluigi has contacted Stefano for an update on the collimation layout in the matching section. A student has started to work on that and a first update is expected in few months.
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Preliminary estimates for the beam-beam effects. S. Valishev
The benchmarking of Lifetrac against Sixtrack has continued and good agreement between the results of the simulations provided by the two codes is observed although some discrepancies remain. Excellent progress has been done for the 6D lens in Sixtrack where some bugs have been fixed and the two codes provide now similar results for round and flat optics. Tatiana pointed out that 30 slices are used for Sixtrack simulations since that does not impact much the simulation time. Sasha noted that this value should be acceptable for all studied cases and for a given case the minimum number of slices that should be used increases with the order of the resonances that can be observed with frequency map analysis.
For the nominal scenarios and * levelling in 3 Interaction Points (pessimistic scenario) the Dynamic Aperture (DA) is larger or equal to the collimator aperture (it must be noted that the DA is expressed in unit of beam sigma - emittance of 2.5 m) even for reduced crossing angle at the beginning of the fill and crossing angle compensation by crab cavities. A few simulation cases were studied representing several scenarios without imperfections and 3 collision points at the beginning of fill, middle of fill, and end of fill. Tatiana commented that the effects of the multipolar errors at 15cm are marginal on beam-beam while this is not the case for 10 cm. Lifetrac predicts luminosity lifetimes of 80 hours (no burn-off or IBS included) at the beginning of the fill. Gianluigi asked what is the expected luminosity lifetime for the 2012 LHC beam parameters. Sasha will verify and provide the data for benchmarking purposes. Action: Sasha.
Strong-strong simulations conducted by Ji Qiang predict qualitatively bunch shortening for bunches with larger emittance. The effect is more visible for large chromaticity and/or larger momentum spread. The simulated bunch shortening rate is comparable with that observed in 2012. Massimo asked what is the explanation for the observed effect. Sasha replied that beams with larger emittance could sample the non-linear part of the head-on beam-beam field and this could explain why the bunch shortening effect is more pronounced for beams with larger emittance.
The option of e-lens as beam-beam compensator is being studied at FNAL. Simulations indicate a clear improvement in DA with the beam-beam wire compensator. Gianluigi asked what is the expected reduction in the beam-beam long range separation that can be achieved with the beam-beam wire compensator. Sasha replied that he expects a reduction of 3 sigmas. Oliver asked what is the beam wire separation in the simulations. Sasha replied that the separation is 9.5 beam sigmas (corresponding to 8 collimation sigma). It was noted that this is not compatible with machine protection (unless an electron beam is used for that purpose) as this is not in the shadow of the tertiary (and not even of the secondary) collimators.
Gianluigi noted that it would be good to show the possible reduction in long range beam-beam separation with simulations for the US-LARP meeting.
Updates from Task Leaders – O. Brüning, M. Giovannozzi, E. Métral, T. Pieloni, A. Valishev
Task 2.2 and 2.3 (Massimo)
Discussions on the possibility of operating the MQY at 200 T/m if cooled to at 1.9K are ongoing. Gianluigi asked if this point could be brought up in the HL-TC. Oliver replied in the affirmative.
Task 2.4 (Elias)
Elias presented the impact of the operation of a second harmonic RF system on transverse beam stability. Both bunch shortening and lengthening mode have been studied. The latter would be required for pile-up density reduction and possibly for IBS growth rate reduction. The threshold for the onset of TMCI is reduced significantly at zero chromaticity although this can be suppressed with moderate values of the chromaticity.
Task 2.5 (Tatiana)
Studies are progressing on the dependence of the dynamic aperture on the crossing angle for different values of the *.
The 6D beam-beam lens is being implemented in the COMBI. Bunch by bunch difference studies are ongoing and can be presented in two weeks. Tatiana is discussing with the BI colleagues the possibility of using beam transfer function measurements for qualifying beam-beam effects and in particular the effect of the long range wire compensator.
Task 2.6 (Oliver)
Nothing to report.
Next meeting will take place regularly on Friday 25th April 2014 at 16:00.
Reported by Gianluigi and Riccardo.
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